The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Recently, increases of oil price has triggered more competition for enhancing fuel efficiency of a vehicle.
In this sense, research on an automatic transmission has been performed to provide better drivability and to improve fuel efficiency by achieving more shift stages.
In order to achieve more shift stages for an automatic transmission, the number of parts is typically increased, which may negatively affect installability, production cost, weight and/or power flow efficiency.
Therefore, in order to improve fuel efficiency of an automatic transmission by more shift stages with reduced number of parts, an eight-speed automatic transmission has been recently introduced, and a planetary gear train for an automatic transmission enabling more shift stages has been developed.
An automatic transmission of eight or more shift-stages typically includes three to four planetary gear sets and five to six control elements (frictional elements), and may easily become lengthy, thereby deteriorating installability.
In this regard, disposing planetary gear sets in parallel or employing dog clutches instead of wet-type control elements is sometimes attempted. However, such an arrangement may not be widely applicable, and using dog clutches may easily deteriorate shift-feel.
In addition, we have discovered that a recent eight-speed automatic transmission typically shows a gear ratio span between 6.5 and 7.5, and improvement for better fuel consumption is limited.